It is known that blood is a very sensitive liquid living tissue and that it is easily altered upon contact with chemical substances or upon an exposure to mechanical constraints, for example upon shearing; it coagulates upon a contact with most of the inert materials or upon stases. In reality, there are very few hemocompatible materials and most of them need anticoagulant taking by a patient bearing such a hemocompatible material.
It is known moreover that (see for example document U.S. Pat. No. 5,135,539) there exist cardiac prostheses wherein the artificial ventricles comprise flexible membranes of a hemocompatible material being operated by fluid impulses so as to activate blood. In such case, the hemocompatibility of said membranes is particularly critical, due to the fact that the membranes are mobile and in contact with a complex and often turbulent blood flow.
In the state of art, substantially hemocompatible materials are known, which are either synthetic or from biological origin.
Synthetic materials are generally elastomers of polyurethane or silicone; they are used either with a smooth surface, so as to reduce the platelet or blood adhesions, or with a porous surface, so as to allow the adhesion of a biological layer adapted to serve as an interface with blood. Such synthetic materials present good qualities of flexibility, imperviousness and deformability, but need the use of anticoagulants.
The materials of biological origin are animal tissues or are reconstituted from biological material, for instance collagen. Tissues of animal type must be chemically fixed (the most often with glutaraldehyde) when they aim to be implanted into the human body, so as to avoid immunological reactions. Such so-treated biological materials generally present excellent hemocompatible properties and do not need anyway the use of anticoagulants by the patient, but they are absolutely not impervious.
On the contrary, synthetic materials, so-called hemocompatible and implantable, generally have interesting mechanical and imperviousness characteristics, but are only tolerated in the blood flow with the help of a strict anticoagulation.
In order to be in a position to take advantage of the good mechanical and imperviousness properties of the synthetic materials and of the good hemocompatibility characteristics of the materials of biological origin, the document U.S. Pat. No. 5,135,539 provides a superposition of a synthetic material membrane and a biological origin membrane. However, such an arrangement leads to the formation of an intermediate chamber between said membranes, which can be the object of undesirable infections or liquid collections.
In order to remedy such disadvantages of the above mentioned state of the art, European patent EP 1,785,154 describes a hemocompatible material comprising a resistant, flexible and impervious synthetic material, for instance being made of an elastomer of polyurethane or silicone, and an animal biological tissue, for example, from animal pericardium, said biological tissue being made integral with said substrate through a dispersion of the constituent material of said substrate in a solvent, said constituent material impregnating said animal biological tissue.
Thus, thanks to the document EP 1,785,154, a composite material is obtained, the hemocompatibility of which is provided by the biological tissue, whereas the mechanical resistance and the imperviousness are brought by the synthetic substrate. It should moreover be noticed that, when said biological tissue consists in an animal pericardium, for example a bovine pericardium, said biological tissue is itself resistant and participates in the mechanical resistance of said composite material.
In order to allow the integration of said animal biological tissue onto said synthetic substrate through said dispersion, it is indispensable to dehydrate said animal biological tissue. To do so, the document EP 1,785,154 envisages to lyophilize said animal biological tissue, which not only dehydrates the latter, but also allows to hold the tridimensional structure of said biological tissue after dehydration. Indeed, when a biological tissue is dehydrated on ordinary conditions, the constituent collagen fibers come in contact with each other and irreversible chemical reactions occur, making the subsequent rehydration of the biological tissue impossible. On the contrary, lyophilizing allows the structure of the animal biological tissue to be immobilized by freezing, then the water to be removed at very low pressure by sublimation, thus with no possibility of moving or rearranging fibers. However, the period of the lyophilizing step is long (at least 96 hours) and it is necessary to implement a specific and costly infrastructure. Furthermore, The lyophilizing procedure is delicate, since dehydration must not be complete, otherwise the animal biological tissue would be irremediably damaged. Now, the dehydration efficiency depends on the thickness and the nature of said biological tissue, so that it is difficult to master and the lyophilizing step is inevitably accompanied with a quite high percentage of waste. Moreover, such incomplete dehydration of the animal biological tissue makes the latter instable, so that the storing and transport thereof on the lyophilized condition are complex and need to be put under vacuum.
The present invention aims at remedy such drawbacks.